The invention relates generally to geophysical surveying technology, and more particularly to a method for locating and identifying underground structures with a horizontal borehole to a surface of the earth transmission path geometry.
The utilities industry faces a significant problem in protecting underground utilities from damage during excavation. While many methods exist for locating underground utilities, no existing technology is adequate for crowded urban environments. The existing methods have great difficulties with heterogeneous soil conditions, asphalt and concrete surface coverings, nearby metallic objects, and deep pipes. Often, they also create a significant disruption of traffic flow at the surface.
One simple approach currently employed involves the excavation of exploratory boreholes to determine the presence or absence of underground utilities. At best, this process is slow and cumbersome in that a great number of exploratory boreholes must be drilled to locate utilities with adequate spatial resolution. At worst, it is destructive when the utilities to be located and then avoided are inadvertently damaged by the exploratory borehole itself.
Non-destructive digging techniques, such as hand excavation and vacuum drilling with compressed air or high pressure water jets, do alleviate the possibility of exploratory damage, but are inefficient and costly. In addition, the need for placement of surface excavation equipment over the entire area of interest creates significant disruption at the surface.
Thus, while exploratory excavation does provide results with a high level of certainty, it is cumbersome, inefficient, costly, and highly disruptive. Accordingly, other non-destructive methods not requiring excavation have also been proposed, most of which are based on geophysical surveying techniques.
These methods typically involve the introduction of a signal into the region of ground being explored. By noting the extent to which this signal is reflected, transmitted, or absorbed, and by considering the physical properties of soil and any other materials that may lie beneath the surface, the location and in some instances the shape and composition of the underground utilities can be inferred.
Several approaches have relied on acoustic signals for this purpose, as evidenced by a number of United States Patents filed in this area (see References). The most common approach is the use of ground penetrating radar (GPR), as in the systems offered by PipeHawk PLC, of Church Crookham, United Kingdom (http://www.pipehawk.com), and Geophysical Survey System, Inc., of Salem, N.H. (http://www.geophysical.com/). More variations of the GPR approach are described in a large number of United States patents (see References).
The greatest weakness of most such systems is a reliance on a surface transmission and surface reception geometry. Inherent in such approaches is a rather shallow maximum depth from which a return signal of adequate strength can be obtained. Accordingly, many of these technologies experience difficulty in situations where deeply buried utilities must be located. These problems are aggravated when probing through asphalt or concrete surface coverings, in heterogeneous soil conditions, or near metallic objects.
U.S. Pat. No. 4,161,687 granted to Lytle describes a system with a possible alternative geometry. The disclosed approach involves xe2x80x9clowering an electromagnetic signal transmitting antenna down one borehole and a receiving antenna down another, the ground to be surveyed for anomalies being situated between the boreholes.xe2x80x9d Such an approach, however, requires the excavation of two boreholes.
U.S. Pat. No. 4,751,688 granted to Paulsson describes a downhole seismic source that produces a seismic signal that can be measured at another borehole or perhaps the surface. However, the device is designed for use in vertical boreholes, xe2x80x9cparticularly cross borehole measurements and vertical seismic profilingxe2x80x9d.
What is desired is a method, using advances in computing and horizontal boring technology that produces a highly accurate image of underground utilities, with minimal disruption to activities on the surface.
The method according to the present invention begins by using a horizontal boring tool to excavate a substantially horizontal borehole that crosses under the surface of the earth at a specific depth. The depth is chosen so that the horizontal borehole passes safely below any existing utilities. In the preferred embodiment, one emitter and one receiver are required. The emitter is placed underground and the receiver on the surface. While the emitter is moved along the passage of the borehole, the receiver is moved along a pathway along the surface of the earth. The received signal, which reflects the transmissive properties of the medium between the emitter and the receiver is recorded; and at the same time, measurements of the emitter""s and the receiver""s positions are taken. The recorded signal data, along with the information representing the measurements, is then processed and transformed into a model of the sub-surface.
The method according to this invention is a direct path method, based on the transmissive, rather than reflective, properties of the surveyed medium. It may be used with a variety of emitter types, including electromagnetic pulse, x-ray, gamma ray, seismic, etc. It may also be used in a multi-sensor mode to combine data from more than one type of emitter. Because the system does not rely on reflections, attenuation should be less of a problem than it is in conventional systems. The direct signal is much stronger than the reflected signal, as it only has to travel one pass through the medium. Also, because the emitter is underground, a more powerful emitter can be used and it is easier to efficiently couple it with the soil.
As an alternative, instead of moving a single emitter, an array of emitters is positioned along the passage of the horizontal borehole. In this way, regardless of the motion pattern of the receiver, the time required to obtain data may be reduced.
In another embodiment, an emitter is moved along a pathway along the surface, while an underground receiver is moved along the passage of the horizontal borehole. As an alternative, instead of moving a single receiver, an array of receivers is used.
In another embodiment, an array of emitters is used to transmitting a signal to an array of receivers, involving the same principle as described above.
In each case, the desired result is a set of data summarizing the transmissive properties obtained from various underground locations, along the borehole, relative to various surface locations, either along a line, in a grid, or in any other geometric configuration.